1. Field of the Invention
This invention relates generally to the field of extractive metallurgy, and, specifically, to the production of niobium metal (Nb) from its precursor compound metallurgical grade ferroniobium (FeNb) or any similar compound ranging from 20% Nb up to 95% or even higher Nb contents.
2. Description of the Previously Published Art
Niobium is a heat resistant, corrosion resistant metal with many specialized applications. Niobium and its alloys are used in sodium vapor lamp filaments, nuclear reactors, rocket nozzles, superconducting alloys, jewelry, and jet engine afterburners, among many other uses.
Most metallic Nb is produced according to the process of aluminothermic reduction (ATR) of niobium pentoxide, Nb.sub.2 O.sub.5, with powdered aluminum (Al), according to the reaction EQU 3Nb.sub.2 O.sub.5 +10Al=&gt;6Nb+5Al.sub.2 O.sub.3. (1)
ATR is an extremely exothermic reaction and it generates sufficient temperature to achieve the melting point of Nb at 2415.degree. C. The slag formed is slightly substoichiometric Al.sub.2 O.sub.3 and is very viscous. The high viscosity makes separation of the slag and metal difficult resulting in lowered recovery of the Nb metal. For this reason, a slag modifier which will greatly reduce the viscosity of the slag is added in the form of BaO.sub.2 (barium peroxide), along with sufficient Al to react with all of the oxygen provided by the BaO.sub.2. The reaction is: EQU 2Al+3BaO.sub.2 =&gt;Al.sub.2 O.sub.3 +3BaO
The barium must be used as a peroxide to provide sufficient heat to bring the BaO up to the temperature of the other reactants. The metal and slag then separate according to their different liquid densities with the metal on the bottom and the lighter slag on top. The amount of BaO.sub.2 is determined by selecting a low melting point in the Al.sub.2 O.sub.3 --BaO.sub.2 phase diagram which will facilitate the separation of slag and metal. The slag solidifying last and the metal first after they have separated.
The slag layer contains mostly Al.sub.2 O.sub.3 with barium oxide, BaO. Of economic import, BaO is toxic, and thus the slag must be treated at some expense prior to disposal. The Nb layer, referred to as a Nb thermite "derby," contains several weight percent Al, up to several thousand ppm oxygen, and trace amounts of other impurities, all of which render the derby material friable and unsuited for conventional metallurgical processes. Electron beam melt refining is the accepted practice to purify the niobium and make it ductile and suitable for fabricating into useful shapes as well as for alloying.
In the electron beam melt refining process, the derby material is hung vertically in an electron beam melting furnace, and exposed to intense electron bombardment under high vacuum conditions. The electron beam causes the material to melt and drip into a water cooled copper ingot mold, thereby forming a consolidated ingot of Nb.
Under the high vacuum conditions within the furnace chamber, volatile impurities within the derby are vaporized at the temperature of liquid Nb. These impurities evaporate away from the pool of liquid Nb maintained in the ingot mold, and condense on the interior walls of the furnace chamber. Iron and some aluminum are removed by simple vaporization in this manner. Nitrogen is removed as gaseous nitrogen. Oxygen is removed as various niobium oxides, such as NbO or Nb.sub.2 O in the form of carbon monoxide, CO (if sufficient C is present) or in various suboxides of Al. Many of these compounds are non-stoichiometric, and are formed and liberated only in the intense heat and high vacuum conditions present within the furnace of about 2450.degree. C. and about 0.1 millitorr, as known to those skilled in the art.
The removal of volatile impurities is an exponentially decaying process, in which large amounts are removed during the first electron beam refining melt, but successively smaller amounts are removed during subsequent melts, in which the ingot from a previous melt is remelted to form a new ingot. Experience has shown that typical Nb thermite derbies must be subjected to the electron beam melt refining process several times before Nb of adequate purity is obtained. ATR and electron beam melt refining are described in the book Extractive Metallurgy of Niobium, by C. K. Gupta and A. K. Suri, CRC Press, 1994 (referred to as "Gupta" below). ATR is additionally taught in U.S. Pat. No. 2,789,896; Canadian patent 620,036; and the article "Metallothermic Reduction of Oxides in Water-Cooled Copper Furnaces," Transactions of the Metallurgical Society of AIME, Vol. 239, pp. 1282-1286.
This method of producing pure Nb is very expensive, but to date has been the only method proven capable of producing adequately pure Nb. A major portion of the expense lies in the production of the precursor compound niobium pentoxide, Nb.sub.2 O.sub.5.
Most Nb is extracted from Brazilian pyrochlore, a complex ore containing iron, niobium, tantalum, oxygen, barium, phosphorus, silicon, titanium, sulfur, manganese, and other elements. Pyrochlore is subjected to various treatments to concentrate the niobium values and is then subjected to an electric arc smelting process using aluminum as a reductant and iron oxide additions to produce metallurgical grade ferroniobium (FeNb), a nearly stoichiometric intermetallic compound of iron and niobium, which is typically on the order of 65% Nb by weight. Of great significance, FeNb contains appreciable amounts of P and Si, which elements are not always appreciably removed by the process of electron beam melt refining. For this reason, FeNb has not itself, in the past, been successfully subjected to electron beam melt refining as a method of producing pure Nb.
FeNb is sold directly to the steel industry, where it is an important additive for alloying purposes. FeNb is also the starting point for the production of pure Nb.sub.2 O.sub.5 to be used for the production of ATR pure Nb. There are several methods by which FeNb may be converted to Nb.sub.2 O.sub.5, two of which are described below. One method is the chlorination of FeNb at a temperature between 500 and 1,000.degree. C., which yields FeCl.sub.3 and NbCl.sub.5. The FeCl.sub.3 is removed by passing the mixed chloride vapors through a bed of NaCl, which forms a eutectic composition with FeCl.sub.3 and thereby removes it. The NbCl.sub.5 is condensed to a solid and after several additional processing steps is ultimately calcined in a rotary kiln to produce pure Nb.sub.2 O.sub.5 with only trace amounts of P and Si. This method of producing Nb.sub.2 O.sub.5 from FeNb is also described in the reference Gupta.
A more recent method is to nitride FeNb by reaction with nitrogen at 1,000.degree. C., producing ferroniobium nitride. An acid solution is then used to dissolve the iron content (also reducing the P and Si content), yielding niobium nitride, NbN. This compound is then calcined to produce Nb.sub.2 O.sub.5, or NbN may be subjected to a high temperature vacuum heat treatment, which removes enough nitrogen that the product may be subjected to electron beam melt refining without having to undergo ATR. The production of ferroniobium nitride is described in U.S. Pat. No. 5,322,548, Kieffer et al.; by Guidotti et al. in U.S. Pat. No. 3,775,096; by Sommers et al. in U.S. Pat. No. 5,284,639; and in Gupta. Decomposition of NbN to produce pure Nb is taught by Guidotti et al in U.S. Pat. No. 3,775,096; in U.S. Bureau of Mines Report of Investigations 8103, and in Japan Patent JP 0339,426 [91 39,426]. Although the nitride route may offer some cost advantages over the chlorination method, the production of ferroniobium nitride is also relatively expensive.